1. Field of Invention
The present invention relates generally to a semiconductor wafer chemical mechanical polishing system. More specifically, the invention relates to a loadcup for positioning a workpiece or semiconductor wafer into a retaining ring of a chemical mechanical polishing system.
2. Background of Prior Art
In semiconductor wafer processing, the use of chemical mechanical polishing, or CMP, has gained favor due to the enhanced ability to stack multiple feature layers on a semiconductor workpiece, or wafer. As the demand for polishing wafers as part of a semiconductor fabrication process increases, the requirement for higher processing rates while minimizing risk of wafer damage and contamination has correspondingly experienced greater urgency for improvement.
Two such CMP systems that address these issues are described in a patent to Perlov et al. (U.S. Pat. No. 5,804,507, issued Sep. 8, 1998) and in a patent to Tolles et al. (U.S. Pat. No. 5,738,574, issued Apr. 15, 1998). The Perlov et al. and Tolles et al. patents are hereby incorporated by reference. The Perlov et al. and Tolles et al. patents disclose a CMP system having a polishing apparatus that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station.
A pedestal within the transfer station rises to receive the wafer from the robot. The wafer is released from the robot and secured, or chucked, to the pedestal by a vacuum chuck. The pedestal is then retracted into the transfer station wherein the wafer is released temporarily from the pedestal so that a plurality of alignment jaws may manipulate the wafer into a position centered upon the pedestal. The wafer is then re-chucked to the pedestal and the pedestal rises to engage and transfer the wafer to a retaining ring of a polishing head. The alignment of the wafer to the center of the pedestal ensures that the wafer will be properly fixtured into the retaining ring.
After the wafer is fixtured into the retaining ring, a carousel delivers the retaining ring and wafer to a polishing station. After completion of the polishing process, the wafer is returned back through the pedestal to the proper cassette located in the bath.
Although this process has proven to be an efficient mechanism for loading and unloading wafers into and out of the chemical mechanical polishing system, an improvement has been identified, which could enhance the transfer of the wafer into the retaining ring, reducing the risk of wafer damage and contamination, while increasing the centering accuracy of the wafer on the pedestal chuck and to the retaining ring, and improving the reliability of wafer loading, minimized wafer breakage, and lowered particulate contamination to the wafer in process.
More specifically, the process of aligning the wafer to the center of the pedestal requires raising and lowering the pedestal. Elimination of the need to lower the pedestal into the transfer station would improve the routing time require to process each wafer, and yield a corresponding increase in wafer throughput. Additionally, as the process of alignment requires both contact of the alignment jaws to the wafer and the physical sliding of the wafer upon the pedestal, both actions incur the possibility of wafer damage and particulate contamination. This is of special concern since any abrasion, scratching or other damage would be occurring on the feature side of the wafer.
Therefore, there is a need in the art for an apparatus that facilitates wafer transfer into a CMP retaining ring wherein centering accuracy of the wafer to the pedestal chuck and the retaining ring is improved, while minimizing wafer breakage and particulate contamination.